High molecular weight linear and branched polyolefins can be added to semi-crystalline polyolefins to increase their toughness and strength. The addition raises the inter-crystalline connectivity or the tie chain concentration of semi-crystalline polyolefins. High molecular weight linear polyolefin contributes to the creation of tie chains due to their inability to completely relax and organize their long backbones during the formation of chain-folded crystallites. Short-chain and long-chain branches in branched polyolefins also contribute to the formation of tie chains as the branches cannot be incorporated into growing crystallites. Typically, short-chain branches are introduced into high molecular weight polyolefins by using higher carbon number alpha olefin comonomers, such as butene, hexene, or octene, copolymerized with ethylene or propylene. Ultra high molecular weight and high molecular weight polyolefins are commonly linear.
In semi-crystalline polyolefins, the stack crystallites self-assemble into a meso structure of spherulites, cylindrilites, or fibrils. Long chain branches can provide connectivity between these meso structures. A preferred type of long-chain branching for enhancing toughness of a semi-crystalline polyolefin without compromising processability, stiffness, and tear balance is addition of a multi-arm star polymer. The long chain branches of a star polymer can provide connectivity between spherulites or other meso structures. A star typically has a lower impact on the polyolefin processability compared to other branch types such as comb and dendritic architectures. Being a relatively symmetric molecule, a star polymer can resist uni-directional orientation and thus minimize the preferential uniaxial stress-induced crystallization during processing. The uniaxial alignment of crystallites in a semi-crystalline polyolefin can result in final products exhibiting unbalanced tear strengths.
It would be desirable to have an additive for ethylene polymers that would enhance their impact toughness. It would also be desirable to have an additive that provides enhanced impact toughness without substantially diminishing mechanical modulus and shear viscosity.